Human HIV infection is a disease which evolves over a long period of time with a progressive loss of CD4+ T cells. The loss of CD4 T cell function both qualitatively and quantitatively diminishes the capacity of the host to respond to a variety of infections. Finding ways to prevent the gradual loss of CD4+ T cells has been difficult to achieve, although recent evidence indicates that cytokine therapy (IL-2) can increase CD4 T cell count. There has also been interest in studying the qualitative function of CD4+ T cells as the course of disease progresses. Some studies using PBMCs have suggested that a switch in lymphokine production occurs from Th 1 to Th 2 over the course of disease and this precludes a more rapid clinical deterioration. Moreover, it has been suggested that responses to specific antigen can be restored in vitro by culturing cells in the presence of cytokine such as IL-12. The following studies will examine the functional capabilities of CD4+ T cells from patients infected with HIV in response to various antigens. These studies will try to provide new insights as to how the CD4+ T cells are regulated and whether functional changes can be achieved through the use of cytokines. Our initial studies showed that IL-15 enhanced the proliferative response in a dose dependent manner from PBMCs of HIV infected individuals when stimulated by polyclonal mitogen, tetanus toxoid, or HIV specific antigen. The effects of exogenous IL-15 are substantially diminished by adding a neutralizing antibody to the beta chain of the IL-2 receptor. Moreover, the ability of IL-15 to increase proliferation is enhanced by the presence of endogenous IL-2 produced in the cultures. Addition of IL-2 or IL-15 to short term in vitro cultures of either PBMCs or CD4+ T cells had little effect on IL-2, IL-4 or IFNgamma production. By contrast, IL-12 caused substantial enhancement of both IL-2 and IFNgamma production from these cultures. These results show that IL-12 and Il-15 have distinct effects on the in vitro immune responses using cells from individuals infected with HIV. While cytokines can have potent effects on the immune response, recent work has demonstrated that co-stimulatory molecules can also profoundly effect the immune response. The interaction of CD40 ligand on activated CD4+ T cells with CD40 on B cells has been shown to be critical in allowing B cells to make immunoglobulin such as IgG. Furthermore this interaction is also important for T cell activation and development of a Th 1 response. Our studies will focus on how CD40 ligand/CD40 interactions control CD4+ T helper responses and the mechanism by which this occurs.